Micro machines are intensively developed recently, such as ultrasmall piezoelectric actuators, motors and pumps, by utilizing the micro machine technology. Among them, the piezoelectric actuator is developed for ink jet printer or use in head positioning in a disk recording and reproducing apparatus (hereinafter called disk apparatus) in which positioning of high precision is demanded in a miniature region.
For use in a disk apparatus, for example, it is developed as follows. Usually, a head for recording and reproducing information in a disk-shape recording medium (hereinafter called disk) is mounted on a head slider, and is attached to an actuator arm. By oscillating this actuator arm by a voice coil motor (hereinafter called VCM), it is positioned on a desired track position on the disk, and information is recorded or reproduced by the head. As the recording density is enhanced, however, sufficient positioning precision can be no longer assured by the conventional VCM alone.
Therefore, in addition to the positioning means using the VCM, it has been proposed to position at higher speed and higher precision by finely driving the head slider by using fine positioning means by a piezoelectric actuator using a piezoelectric element (Ultrahigh TPI and piggyback actuator, IDEMA Japan News No. 32, pp. 4–7, International Disk Drive Association).
The piezoelectric elements used in such piezoelectric actuator may be manufactured by green sheet laminating method, multilayer ceramic method formed by thick film multilayer lamination, or thin film method fabricated by thin film technology.
In the piezoelectric actuator using these piezoelectric elements, displacement of the piezoelectric element deteriorates in the course of time due to impression of voltage. In order that the piezoelectric actuator may operate stably without such aging or deterioration, it is required to suppress changes of initial polarization characteristic of the piezoelectric element. Various measures of stabilization of polarization are devised in the manufacturing process.
For example, to stabilize the polarization characteristic, in a piezoelectric actuator manufactured in a conventional manner, sinusoidal waves oscillating from 0 voltage to maximum operating voltage of the piezoelectric actuator are applied for more than 1 hour, preferably about 10 hours, and the piezoelectric actuator is stabilized (Japanese Patent Publication No. 2814607, FIG. 3).
In this method, prior to use of the piezoelectric element, it is stabilized by applying sinusoidal waves ranging from, for example, 0 V to 150 V. However, when such high voltage is applied for 1 hour, or preferably as long as 10 hours, the insulation resistance may deteriorate in the case of the piezoelectric element fabricated by the thin film technology. Further, since such method is intended to stabilize by slightly decreasing initial displacements of the piezoelectric actuator, and as compared with the displacement in the initial polarization state, the displacement is smaller in the actual operating state, and the performance as the piezoelectric actuator is slightly lowered.
Concerning an asymmetric hysteresis characteristic of a piezoelectric thin film, it is proposed to form a platinum (Pt) film of (100) azimuth on a single-crystal magnesium oxide (MgO) substrate of (100) azimuth, and form a (001) oriented lead zirconic titanate (PTZ) film thereon by sputtering method (Appl. Phys. Lett. 70 (11), 1378–1380, 17 Mar. 1997, “Piezoelectric properties of c-axis oriented Pb (Zr, Ti) O3 thin film”). This publication, however, teaches nothing about driving method as piezoelectric actuator for stabilizing polarization of piezoelectric element or preventing deterioration of insulation resistance.